Seamless precipitation prediction skill in a global model: Actual versus

potential skill

Matthew Wheeler1, Hongyan Zhu1, Adam Sobel2, and Debra Hudson1

The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

1

2 Columbia University, New York, USA

Motivation• We began this work with the expectation that the extratropics are more predictable at

short lead times (~days), while the tropics are more predictable at long lead times

(months to seasons). But where does the cross-over occur?

• What can we learn about the sources of predictability from a global analysis of model

prediction skill for different seasons?

Additional benefit:

• Our development of a simple method for comparing prediction skill across a large

range of timescales that is applicable around the globe.

The essence of our approach is:1. Use precipitation for fair global comparison.

2. Compute skill globally for a large range of lead times.

3. As we increase the lead time, we also increase the time-averaging window for a seamless transition from weather to climate.

Schematic of window/lead definitions

Data and Method

a. POAMA-2 ensemble forecast system

T47L17 atmosphere; 0.5-2º L25 ocean; and land.

Initialized with realistic atmospheric, land, and ocean initial conditions.

Coupled breeding scheme to produce a burst ensemble of 11 members.

3 versions of the model to provide in total 33 members.

Hindcasts from the 1st, 11th, and 21st of each month (out to 120 days).

b. Observations

GPCP daily precipitation (blended station and satellite).

1º grid converted to POAMA grid.

We use 1996 to 2009 for this work.

c. Measure of prediction skill

We tried different verification measures (ROC score, Brier score, correlation skill).

In the end we chose the simplest: the correlation of the ensemble mean.

We use two versions:

CORt - using total precipitation values

CORa - using anomalies with respect to separate climatologies for the

hindcasts and observations.

CORt is more usual for weather prediction; CORa is more usual for seasonal prediction.

The correlations are computed over time using data from multiple verification times.

Separately for each lead time and each grid point.

Separately for DJF (n=117) and JJA (n=108).

Only CORa is shown in this presentation.

Remember these window/lead definitions

Like what others call "month 2"

Initial condition

4w4w

CORa (i.e. removing the influence of the climatological seasonal cycle)

1d1d: Extratropics better than tropics; winter extratropics better than summer.4w4w: ENSO dominates.

Zonally-averaged CORa

The peak in skill at the equator is apparent at all lead times.Extratropical skill drops rapidly from 1d1d to 1w1w and then levels-off.

Skill in tropics (10ºS-10ºN) overtakes skill in extratropics for 4d4d in DJF and 1w1w in JJA.

CORa: plotted as a function of the log(time)

Comparison with persistence

An important component of predictability is the prediction skill that can come from persistence. What is its contribution here?

Use the most recent observed precipitation anomalies to predict future anomalies.

1 week average

1 day average

Initial condition

1d1d CORa for persistence (top) model (bottom)

Tropics are generally more persistent than extratropics, but model forecasts convincingly beat persistence almost everywhere.

Persistence of ENSO is obvious, but model still beats persistence in most locations, including around Australia and the US west coast in DJF.However, the model does not get the persistence skill around the sea-ice edges, because it currently uses prescribed climatological sea ice.

4w4w CORa for persistence (top) model (bottom)

Comparison with Potential (or Perfect) Skill:using the assumption that one ensemble member is truth

(All seasons)

Actual skill

Potential skill(perfect model assumption)

Can interpret the difference as either "room for actual improvement" or "too little spread". Here we see the greatest difference for the tropics at the shorter lead times.This points the finger at the tropics (and moist convection) as our biggest handicap.

Conclusions• For the shortest (1 day) lead time, actual skill is greatest around 40-

60º latitude, lowest around 20º, and has a secondary local maximum near the equator.

• Short range extratropical skill is greater in winter than summer.

• Skill from ENSO apparent even at short lead times.

• Tropical skill (within 10º of the equator) over-takes extratropical skill for lead/window times of ~4-7 days.

• The model almost everywhere beats persistence, especially for short lead times.

• Actual vs. potential skill differences point to the tropics at short leads as requiring the most attention (more actual skill -or- more spread).

• Finally, we advocate the usefulness of our seamless verification approach, showing skill across a large range of timescales.

m.wheeler@bom.gov.au

Extra slides

Comparison of CORa maps with CORt maps

CORa (i.e. removing the influence of the climatological seasonal cycle)

1d1d: CORa is very similar to CORt.4w4w: CORa is generally smaller than CORt away from the equatorial Pacific.Skill over Australia (e.g. compared with Africa) still looks pretty good!

CORt (i.e. includes seasonal cycle)

High CORt here is evidently due to a good representation of the seasonal cycle.

Using 1-day windows

The usual approach: Fixed time-averaging window of 1 day

Zonally-averaged CORt for 1-day windows

Monotonic decrease in skill with lead time for all latitudes.However, the rate of skill loss is less in the tropics, giving the same general conclusion of a transfer in skill from extratropics to tropics with increasing lead time.

Model CORt for 1-day windows

Cross-over in skill now appears at 4 days (DJF) and 14 days (JJA).A slightly longer value for constant 1-day windows makes sense.

Maps for the intermediate time scales: CORa

CORt

CORt

1d1d: Extratropics better than tropics; winter extratropics better than summer.4w4w: ENSO dominates. Something strange around 50-65ºS in DJF?

Zonally-averaged CORt

ENSO peak at equator is apparent at all lead times.Extratropical skill drops rapidly from 1d1d to 1w1w and then levels-off.

CORt: plotted as a function of the log(time)

Equatorial skill remains almost constant with time.Extratropical skill has a minimum at 2w2w.

Actual versus potential predictability

Comparison with Potential Predictability:Under the assumption that one ensemble member is truth.

ACTUAL

POTENTIAL

Comparison with Potential Predictability:Under the assumption that one ensemble member is truth.

ACTUAL

POTENTIAL